The proposed research is on the bacterial pathogen Coxiella burnetii, the causative agent of the zoonotic disease Q-fever and a pathogen with a remarkably low infectious dose (fewer than 10 bacteria can lead to an infection). This pathogen is able to replicate to large numbers by actively maintaining its acidic vacuole. In order to accomplish this, C. burnetii injects effector proteins across the vacuolar membrane into the cytoplasm of the host cell via the type IV secretion system. Type IV secretion systems are employed by many Gram negative bacteria as a means of interaction with eukaryotic organisms as it allows for the delivery of proteins or DNA across membranes. The proposed research aims to characterize, at the molecular level, a type IV-secreted protein from C. burnetii that was identified as being important in vacuole biogenesis and intracellular replication based on a screen of a recently constructed transposon based mutant library. The project is comprised of two specific aims. The first specific aim is to determine why CBU1780 is required for intracellular replication. To accomplish this, the stage of intracellular infection that is defective in the cbu1780 mutant will be determined. Whether ectopic production of CBU1780 in host cells can restore intracellular replication to the cbu1780 mutant will also be tested. The second specific aim is to discover the biological function of CBU1780. To this end, an RNA interference screen will be carried out to identify host pathways that suppress the intracellular growth defect displayed by the cbu1780 mutant. Host proteins that interact with CBU1780 will be identified. Finally, host pathways that are affected by CBU1780 during infection will be established. Broadly, this research will provide a better understanding of the molecular mechanisms used by C. burnetii, a pathogen that is potentially fatal, to manipulate host pathways to establish an infection.